KeywordsDominant Male Reed Warbler Elephant Seal Great Reed Warbler Scramble Competition
Cohesive social group that is constituted by one or few dominant individuals of one sex (usually males) and, respectively, two or more individuals of the other sex (usually females), and in which dominant individuals actively fight nonmember competitors for all or, at least, the vast majority of the copulations with the group members of the other sex.
The assembly of individuals of one sex (usually females) that is sexually monopolized by one or few individuals of the other sex (usually males).
Polygyny (i.e., the mating combination in which one male is paired to two or more females) is found in different forms, including resource defense polygyny, female defense polygyny, scramble competition, and leks (Emlen and Oring 1977). In these mating systems, high-quality males acquire more than one mate because they are more successful than low-quality males in premating competition. However, this higher success is defined in different ways in each of these polygynous systems. In leks and scramble competition, high-quality males are more successful because they are more effective in finding and/or courting females, but they do not actively restrict the access of other males to females. There are situations, however, in which males do not only compete for finding and courting mates, but also defend females, directly (i.e., female defense polygyny) or indirectly (resource defense polygyny). If a mating system/social group is characterized by a male that prevents other males from accessing his breeding partners, this may be termed a harem. However, through the decades, the term harem has been applied to a variety of contexts, leading to a gamut of different definitions of the concept. The heterogeneous nature of the concept of harem is presented in the following sections.
The Different Applications of the Term Harem
In behavioral ecology studies with nonhuman species, the term harem is most commonly used to designate social groups and mating systems that comprise just one male and more than one female. Social groups of one male and at least two females are found, for example, in arachnids (Gonyleptidae harvestmen), crustaceans (isopod Paracerceis sculpta), insects (stone and tree wetas, and bark beetles), fishes (Centropyge, Serranus), Agama lizards, great reed warbler, pheasants, northern harriers, Saccopteryx bats, elephant seals, fur seals, geladas baboons, hamadryas baboons, gorillas, langurs, pronghorn antelopes, red howler monkeys, red deer, and zebras. In some of these organisms, the harems can be part of a larger grouping of individuals, such as baboon troops. However, for each subgrouping inside the troop, there is a cohesive interaction between a specific male and two or more particular females, which characterizes, under this definition, a harem (Lancaster 1975).
In the examples mentioned above, once females are clustered in space and/or time, a set of males in the population can monopolize sexual partners and prevent other males from mating. For example, female pinnipeds (seals) concentrate on beaches during the reproductive season and, once they are clustered in an area during a specific period, high-quality males can monopolize them, becoming harem bulls that fight off other males. Female monopolization may be achieved by defending territories (e.g., fur seals) or by defending the females per se from the attempts of competing males (e.g., elephant seals), allowing the dominant male to enhance his fitness.
By naming as harems the groups formed by one male and more than one female, it is assumed that one male and two females form a harem. It is also common to describe harems as groups composed of one male and several (or many) females. Yet, some authors define as harems even the groups of one male and just one female, given the potential of the male to acquire additional mates during the same breeding season. Further still, some authors also name as harems those groups composed by few males that, together, monopolize a larger number of females. These male coalitions are found, for example, in animals such as the chimpanzees, feral horses, lions, and the cichlid fish Pelvicachromis pulcher. This joint effort of males may allow the monopolization of females and the maintenance of a harem when there are too many females and/or the females disperse along extensive areas.
Harems can be classified into two categories. Seasonal harems may occur when females are highly synchronized with respect to their sexual receptivity, and their monopolization remains feasible (e.g., red deer, Cervus elaphus). And permanent harems may occur when there is little synchronization in the availability of breeding females, and males are forced to defend their partners constantly (e.g., hamadryas and gelada baboons) (Davies et al. 2012).
Lastly, the term harem can also be used to designate: (a) social groups in which females are the polygamist sex that monopolizes sexual partners (i.e., polyandrous harems; e.g., phalaropes and jacanas), (b) social groups in which an individual of one sex monopolizes conspecific hermaphrodites (e.g., some serranine fishes) (Leonard and Córdoba-Aguilar 2010), and (c) the set of individuals of the most numerous sex in a given social group (i.e., if a single male defends three females, these females can be named his harem).
Harems and Their Evolutionary Implications
When few individuals control access to mates, the intensity of sexual selection upon the controlling sex is large. In pinnipeds, for instance, a large male can control a harem with more than a hundred breeding females. Given that one single male controls all the females at his harem by ferociously displacing challengers, many males will remain unmated. Once few males copulate with several or dozens of females and other males gain few or no mates, there is intense sexual selection (i.e., nonrandom differences in mating success; Kokko and Jennions 2008), and just a few males achieve high fitness.
The great variance in mating success between males tends to lead to the evolution of costly traits that favor males in male-male competition for the access to females. As discussed by Darwin (1871), this may lead to the development of extravagant secondary sexual traits in the nonlimiting sex (males, usually). For instance, there are the great differences in size and aggressiveness between the sexes in pinnipeds and red deer. Red deer also presents great antlers, which are used in fights for access to mates. The development of antlers and the fact that several fights for harem control result in males with permanent wounds, such as broken legs or perforated organs (Davies et al. 2012), show that males can suffer extreme costs in highly polygynous systems.
Under the Shadow of Harem Masters
The great mating success of dominant males that hold harems contrasts with the mean mating success of males that do not possess a harem. Among the latter males, some will engage in alternative strategies to achieve some matings. In the isopod Paracerceis sculpta, while large males hold harems, and defend females from the attempts of challengers, small males use their low detectability (derived from their reduced size) to sneakily enter the harem and mate. Additionally, medium-sized males mimic the size and behavior of females, enter harems, and mate without being displaced by dominant males (Shuster and Wade 1991; Shuster and Wade 2003). Sneaking behavior occurs in several taxa, such as harvestmen, amphipods, decapods, elephant seals, and ungulates.
The existence of harems promotes conflicts between the sexes. These sexual conflicts of interests emerge because, in harems, some males can prevent others from accessing females, which may preclude females the opportunity to choose the identity and number of sexual partners that could maximize their fitness. Moreover, the sexual conflict may emerge in cases in which the dominant harem male kills unrelated offspring, as has been recorded for lions. This happens when the male has recently acquired the harem, and he is assured that he is not the father of the young. This behavior guarantees that the male will not care for the young of a competitor and may bring the female to a fertile state sooner. However, from a female perspective, this behavior only represents a reduction in her reproductive success.
Controversies Regarding the Term
Given the constrain imposed by harems upon the maximization of female fitness, and the existence of male alternative strategies, it is not surprising that, despite the vigilance of the dominant harem male, various females do copulate with other males (e.g., blackbirds, seals, Gunninson’s prairie dogs, Hamadryas baboons, and the cichlid Pelvicachromis pulcher). This leads to sperm competition and reduction of the dominant male’s fitness. If dominant harem individuals are not able to secure the great majority of copulas, it could be argued that they are not monopolizing, sensu stricto, sexual partners.
It also has been pointed out that the term harem brings problems derived from an anthropomorphist approach of studying animal behavior. Early naturalists interpreted animal mating systems by relating them to some specific human social organizations, particularly the harems (Rowell 1991). However, despite the several slightly different definitions of harem in human contexts, the human harem encompasses important political and economic aspects that match no straight analogy in other animals’ harems. In several human societies, the harem represents the political power that derives from the position occupied by a particular man in the social stratification. Moreover, in some ancient human harems, the dominant individual could turn his wives into exclusive sexual partners, once eunuch guards were employed to protect his harem. Thus, the harem owner was assured to father all the children of all the women in his harem. Given these differences between human and nonhuman harems and that anthropomorphism is not an exclusive problem of early naturalist, but also a mistake in which current scientists may incur while studying nonhuman animals, it has been proposed the use of substitutes to the term harem. One such synonym that has been used is the term one-male group (Lancaster 1975).
The use of the term harem has a long history, while at the same time not having a consistent definition. Harems are a widespread form of social organization in animal reproductive systems. Among species, harems differ in the structure of group composition and in which sex is in the role of dominant individual. The variation among different species in aspects such as group composition is very likely one of the main causes for the lack of a consistent definition of what a harem means. Moreover, as the term was first coined to define a form of human social group, carrying human-specific context, it does not readily apply to the reproductive systems of other animals. Presently, there is little consensus as to the definition of the term harem and its applicability to different organisms.
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